This invention relates generally to the field of turbine blades, and more particularly to the field of the repair of the tip portion of turbine blades.
FIG. 1 illustrates a turbine blade 10 as is known in the prior art for use in power generating turbines, such as in the first row of blades of a gas or combustion turbine. Turbine blade 10 includes a blade root 12, an airfoil portion 14, and a tip portion 16. The blade root 12 is designed to be inserted into and retained by a disc on a rotating shaft (not shown) of the turbine. Airfoil portion 14 is shaped to extract energy from combustion gases passing over the airfoil portion 14, thereby imparting rotating mechanical energy to the turbine shaft. For modern gas turbine engines, airfoil portion 14 is designed to include one or more cooling passages formed below the surface of the airfoil for the passage of cooling air necessary to insure the integrity of the blade material in the hot combustion gas environment. Such cooling passages may be formed in a forged blade by a drilling process or may be formed directly in a cast material blade. For cast turbine blades, the cooling passages are formed by supporting a ceramic core within the volume of the mold as the material of the blade is cast. In order to support the ceramic core in its proper position during the casting process, it is necessary to extend a portion of the core to the edge of the casting, thereby creating one or more openings in the tip portion 16 of the as-cast blade. These openings must then be sealed during the fabrication of the blade in order to assure the proper flow of the cooling air within the turbine blade 10. If the size of the opening is sufficiently small, it may be sealed by a weld plug 18 formed on the tip 16 of the blade 10. For larger openings it may be necessary to cover the opening with a cap such as one or more plates 20 in order to seal the opening. U.S. Pat. No. 4,073,599 issued on Feb. 14, 1978, to Allen et al. describes such a blade tip closure design, and it is incorporated by reference herein. Plates 20 are mechanically restrained by the structure of the blade tip 16 and are held in position and sealed by one or more brazed joints 21. It may be appreciated that the assembly and brazing of plates 20 can be a difficult and expensive process. Furthermore, in spite of efforts to maintain the core in its proper position during the casting process, many cast blades are rejected due to a minimum wall violation caused by unintended movement of the core resulting in an end of a cooling passage being located proximate a surface of a tip end of the airfoil of the blade.
Turbine blade 10 is designed to rotate within a casing (not shown). It is important for the blade tip 16 to fit precisely within the casing in order to minimize the passage of combustion gases around the blade tip 16, since such bypass gases impart no energy to the airfoil section 14. The blade tip 16 is provided with a squealer 22 which is a raised lip extending around the periphery of the blade tip 16. Squealer 22 gets its name from the sound that is produced in the event of a mechanical interference between the blade tip 16 and the casing. Ideally the squealer 22 is sized to fit within the casing without rubbing but with a minimum of space there between.
It is known that turbine blades 10 may develop one or more cracks 24 near the tip 16 of the blade 10 due to low cycle fatigue stresses imparted on the blade tip 16 during the operation of the turbine. If a crack 24 extends beyond a critical dimension, the turbine blade 10 must be removed from service and/or repaired in order to prevent catastrophic failure of the blade and turbine. It can be appreciated that a crack 24 may be repaired by removing the material adjacent to the crack 24 to form a crack repair volume, and then filling the crack repair volume with weld metal. However, the presence of braze joint 21 utilized to secure plates 20 in position can complicate the repair process, since weld integrity is adversely affected when applied over a braze material.
In light of the limitations of the prior art designs, it is desirable to provide a method for repairing a cracked hollow turbine blade which overcomes the problems associated with the presence of braze material in the proximity of the cracked area. It is also desired to provide a method of manufacturing a hollow turbine blade that precludes the possibility for a repair in the area of a braze joint. Furthermore, it is desired to provide a turbine blade having improved level of performance to prevent the occurrence of cracks near the blade tip.